The present invention relates to a method and apparatus for directional radio communication in which signals between a first station and a second station may be transmitted only in certain directions. In particular, but not exclusively, the present invention is applicable to cellular communication networks using space division multiple access.
With currently implemented cellular communication networks, a base transceiver station (BTS) is provided which transmits signals intended for a given mobile station (MS), which may be a mobile telephone, throughout a cell or cell sector served by that base transceiver station. However, space division multiple access (SDMA) systems have now been proposed. In a space division multiple access system, the base transceiver station will not transmit signals intended for a given mobile station throughout the cell or cell sector but will only transmit the signal in the beam direction from which a signal from the mobile station is received. SDMA systems may also permit the base transceiver station to determine the direction from which signals from the mobile station are received.
SDMA systems may allow a number of advantages over existing systems to be achieved. In particular, as the beam which is transmitted by the BTS may only be transmitted in a particular direction and accordingly may be relatively narrow, the power of the transceiver can be concentrated into that narrow beam. It is believed that this results in a better signal to noise ratio with both the signals transmitted from the base transceiver station and the signals received by the base transceiver station. Additionally, as a result of the directionality of the base transceiver station, an improvement in the signal to interference ratio of the signal received by the base transceiver station can be achieved. Furthermore, in the transmitting direction, the directionality of the BTS allows energy to be concentrated into a narrow beam so that the signal transmitted by the BTS can reach far away located mobile stations with lower power levels than required by a conventional BTS. This may allow mobile stations to operate successfully at greater distances from the base transceiver station which in turn means that the size of each cell or cell sector of the cellular network can be increased. As a consequence of the larger cell size, the number of base stations which are required can also be reduced leading to lower network costs. SDMA systems generally require a number of antenna elements in order to achieve the required plurality of different beam directions in which signals can be transmitted and received. The provision of a plurality of antenna elements increases the sensitivity of the BTS to received signals. This means that larger cell sizes do not adversely affect the reception of signals by the BTS from mobile stations.
SDMA systems may also increase the capacity of the system, that is the number of mobile stations which can be simultaneously supported by the system is increased. This is due to the directional nature of the communication which means that the BTS will pick up less interference from mobile stations in other cells using the same frequency. The BTS will generate less interference to other mobile stations in other cells using the same frequency when communicating with a given MS in the associated cell.
Ultimately, it is believed that SDMA systems will allow the same frequency to be used simultaneously to transmit to two or even more different mobile stations which are arranged at different locations within the same cell. This can lead to a significant increase in the amount of traffic which can be carried by cellular networks.
SDMA systems can be implemented in analogue and digital cellular networks and may be incorporated in the various existing standards such as GSM, DCS 1800, TACS, AMPS and NMT. SDMA systems can also be used in conjunction with other existing multiple access techniques such as time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA) techniques.
One problem with SDMA systems is that the direction in which signals should be transmitted to a mobile station needs to be determined. In certain circumstances, a relatively narrow beam will be used to send a signal from a base transceiver station to a mobile station. Therefore, the direction of that mobile station needs to be assessed reasonably accurately. As is known, a signal from a mobile station will generally follow several paths to the BTS. Those plurality of paths are generally referred to as multipaths. A given signal which is transmitted by the mobile station may then be received by the base transceiver station from more than one direction due to these multipath effects.
Examples of patents which relate to methods and apparatus for directional radio communication include WO-A-96/37969, EP-A-0647978 and EP-A-0729285. In particular, WO-A-96/37969 discloses base station equipment for receiving a signal by means of an antenna group and including a receiver which monitors at all times the directions from which the best signal components from mobile stations are received. This information can also be used in the base station equipment in the downlink direction. A transmitter unit phases the signal in such a way that the angles of the greatest gain of the or each antenna beam point in the desired directions.
An additional problem is that the direction in which a signal is to be transmitted by the BTS to the mobile station is determined on the basis of the uplink signals received by the BTS from the mobile station. However, the frequencies of the down link signals transmitted from the mobile station to the BTS are different from the frequencies used for the signals transmitted by the BTS to the mobile station. The difference in the frequencies used in the uplink and downlink signals means that the behaviour of the channel in the uplink direction may be different from the behaviour of the channel in the downlink direction. Thus the optimum direction determined for the uplink signals will not always be the optimum direction for the downlink signals.
Embodiments of the present invention seek to provide an improved method and apparatus for directional radio communication.
According to a first aspect of the present invention, there is provided a method of directional radio communication between a first station and a second station, said method comprising the steps of:
receiving at said first station a first signal transmitted from said second station, said first signal being receivable from a plurality of different directions;
determining a principal beam direction from which said first signal is received by said first station;
defining at the first station a plurality of beam directions for transmitting a radiation beam, wherein each of said beam directions is selectable; and
selecting at said first station said determined principal beam direction and at least one other auxiliary beam direction, said at least one auxiliary beam direction being adjacent to said determined principal beam direction and transmitting a second signal from said first station to said second station in said selected beam directions, wherein the transmission power in each of said beam directions is individually determinable and wherein the strength of the second signal transmitted in said at least one auxiliary direction is less than the strength of the second signal transmitted in said determined principal direction.
By using this method, the probability that the signal transmitted by the first station will be received by the second station is increased. As the strength of the second signal transmitted by the second station in the auxiliary direction is dependent on a parameter of the first signal received in that direction, it is possible, for example, if a relatively strong signal is received by the second station in the at least one auxiliary direction to transmit a relatively strong signal to the first station in the at least one auxiliary direction.
In practice, the first signal may be received by the first station from a plurality of directions. Only one of those directions is selected as the determined principal direction. The determined principal direction may be selected in a number of different ways. For example, the determined principal direction may be selected as being the direction from which the first signal is received by the first station with the greatest energy or strength. Alternatively, the determined principal direction may be selected as being the direction from which the first signal is first received by the first station. This corresponds to the signal having followed the shortest path, which may be the line of sight path.
In one embodiment of the present invention, the first signal includes a known data sequence and the method further comprises the steps of correlating the received data signals with the known data sequence in order to obtain the channel impulse response. In one preferred embodiment the received data signals are correlated with a locally generated replica of the known data sequence. The channel impulse response is used to determine which direction is to be the principal direction. For example, the channel impulse response may be determined for each of the channels corresponding to different directions from which the first signal might have been received. The channel impulse response thus received is a measure of the available amount of the desired signal received from the first station. Some parameters of the channel impulse response of each channel may be compared with one another in order to ascertain which of the directions provides the first signal with maximum energy or the minimum delay. The signal with the minimum delay is the signal first received by the first station.
The at least one auxiliary direction may comprise the directions on either side of the determined principal direction.
Preferably, the strength of the said second signal in said at least one auxiliary direction is less than or equal to the strength of the second signal in the determined principal direction.
Preferably, said method comprises the step of defining at the first station a plurality of beam directions for transmitting a radiation beam, wherein each of said beam directions is individually selectable. The transmission power for each of the beam directions may be individually determinable, wherein the transmission power of the beam in the or each auxiliary direction is less than the transmission power in the direction of the principal beam.
In one embodiment of the present invention, the ratio of the strength of the second signal in said at least one auxiliary direction to the strength of the second signal in said determined principal direction is proportional to the ratio of the strength of the first signal received by the first station from said at least one auxiliary direction to the strength of the first signal received by the first station in said determined principal direction. Preferably, these two ratios are equal.
Preferably, if the strength of the first signal received in said at least one auxiliary direction is very much less than the strength of the first signal received in the direction of the determined principal direction, then said second signal is transmitted from the first station to the second station only in said determined principal direction. However, if the strength of the first signal received in said determined principal direction and said at least one auxiliary direction are substantially the same, then the first station is arranged to transmit that second signal in the determined principal direction and in said at least one auxiliary direction at substantially the same signal strength. Thus, when it is determined that the first signal is mostly received from the determined principal direction, then the second signal is only transmitted in that direction. However, if it is determined that the first signal is received with approximately the same strength from two or more directions, then the second signal will be transmitted in those two or more directions with substantially the same strength. There will of course be situations between these two limit cases in which the strength of the second signal in said at least one auxiliary direction will be smaller than the strength of the second signal in the determined principal direction.
Preferably the strength of the second signal transmitted by the first station in at least one of said determined principal direction and the at least one auxiliary direction is dependent on the strength of first signal received by the first station in the respective directions. The strength of the second signal in at least one of the determined principal direction and said at least one auxiliary direction may be dependent on the average strength of a plurality of preceding signals received at the first station from the second station. In one preferred embodiment, the strength of the second signal in one of said determined principal direction and said at least one auxiliary direction is dependent on the strength of said first signal received in the respective direction and the strength of the second signal in the other of said determined principal direction and said at least one auxiliary direction is dependent on the average strength of a plurality of preceding signals received at said first station from said second station in the respective direction. It is preferred that the strength of the second signal in the determined principal direction be based on the strength of the first signal whilst the strength of the second signal in the at least one auxiliary direction be determined on the basis of the average strength of a plurality of preceding signals received from the second station. Thus, the power in the principal direction could be updated on every received signal to rapidly try to follow channel changes affecting the path between the first and second stations In contrast the Dower in the at least one auxiliary direction may respond slowly to changes to try to increase the level of signal received by the second station. This may lead to an increased probability that a signal from the first station will be received by the second station.
A beam in said the or one of the at least one auxiliary direction may overlap a beam defined in the determined principal beam direction. In one proposal, the or one of the at least one auxiliary beam will overlap up to half of the angular spread of the determined principal beam.
Preferably, the method includes the step of determining if the distance of the second station from the first station is below a predetermined value, and if so then the second signal transmitted from said first station to said second station is transmitted with a relatively wide angular spread. In particular, the total angular spread achieved is preferably greater than that achieved when the distance between the first and second stations is above the predetermined value and the principal direction and at least one other auxiliary direction are used for transmitting said signal.
According to a second aspect of the present invention, there is provided a first station for directional radio communication with a second mobile station, said apparatus comprising:
receiver means for receiving a first signal transmitted by said second station, said first signal being receivable from a plurality of different directions;
determining means for determining the principal direction from which said first signal is received;
transmitter means for transmitting a second signal from the first station to the second station, said transmitter means being arranged to transmit a radiation beam in a plurality of beam directions, wherein each of said beam directions is selectable; and
control means for controlling said transmitter means, wherein said control means is arranged to control the said transmitter means to transmit said second signal to said second station in the determined principal beam direction, said at least one auxiliary direction being adjacent to the determined principal direction, wherein the transmission power in each of said beam directions is individually determinable and wherein the strength of the second signal transmitted in said at least one auxiliary direction is less than the strength of the second signal transmitted in said determined principal direction.
The receiver means and the transmitter means may comprise an antenna array which is arranged to provide a plurality of signal beams in a plurality of different directions. The antenna array may comprise a phased antenna array or may comprise a plurality of separate antenna elements each of which is arranged to provide a beam in a defined direction. Two separate arrays may be provided, one to receive signals and the other to transmit signals. Alternatively a single array may be provided both to receive and to transmit signals.
Preferably, the control means is arranged to determine the power levels for said signal in the determined beam direction and at least one other beam direction based on the relative energy levels of the first signal received in said determined beam direction and said at least one auxiliary direction. The relative energy levels may be determined by said control means which correlates at least a portion of the received first signal with a known version of that signal or a portion thereof. As will be appreciated, the first signal may comprise or include a training sequence which is a known sequence of data which is correlated with a reference version of that training sequence which is not distorted in order to determine the channel impulse response. This information may be used to determine the relative power levels and may be used to determine the principal direction.
The transmitter means may be arranged to provide a radiation beam in a plurality of beam directions, wherein each of the beam directions is individually selectable.
Preferably, transmission power for each of the beam directions is individually determinable, wherein the transmission power of the beam in the or each auxiliary direction is less than the transmission power in the principal beam direction.
The present invention is particularly applicable to cellular communication networks. In such networks, the first station may be a base transceiver station whilst the second station is a mobile station. However, it is appreciated that embodiments of the invention may be applicable to any other type of radio communication network such as PCN (Private Communication Networks) or the like.